As drillers create wells to extract fluids from subterranean formations, they often perform a “fracturing” operation in which a fluid is injected into the well bore under high pressure to enlarge any existing fractures in the formation and to create new fractures. The injected fluid often carries entrained particulate matter to be deposited in the fractures, thereby propping them open when the pressure returns to normal. Such fractures substantially increase the permeability of the formation—making it easier for fluid to flow from the formation into the well bore (and vice versa). Fracturing operations are also often employed in injection wells, i.e., wells created to inject fluids into subterranean formations for disposal, storage, or reservoir flooding.
In any case, it is often desirable to confine the effects of the fracturing operation to a bounded region. For example, any fractures that would promote fluid flow between formations are generally undesirable, as such flows can contaminate water tables, relieve reservoir pressures, divert fluids into inaccessible regions, or create other problems. Accordingly, oilfield operators employ models to predict the effects of a fracturing operation and, in some cases, employ micro-seismic detection to monitor fracture evolution during the fracturing operation itself. The seismic sensors are typically positioned in one or more monitoring wells spaced apart from the fracturing well, but in at least one proposed method the seismic sensors are positioned in a concrete annulus around the bore of the injecting well. See, e.g., U.S. Pat. No. 5,503,252 to Withers, titled “System and Method for Monitoring the Location of Fractures in Earth Formations”. Such seismic monitoring methods often perform inadequately in regions having high seismic attenuation or significant seismic interference.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the claims to the particular forms described herein, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the patented claims.